For some time now, foams having a variety of different physical properties have been commercially produced by the isocyanate polyaddition process from compounds containing two or more active hydrogen atoms and especially from compounds containing hydroxyl and/or carboxyl groups, and polyisocyanates, optionally in the presence of water, activators, emulsifiers, foam stabilizers and other additives (cf. R. Vieweg, A. Hochtlen, Kunststoff-Handbuch, Vol. VII Polyurethane, Carl-Hanser-Verlag, Munich 1966). By virtue of this process, it is possible by suitably selecting the components, to obtain foams ranging from rigid foams to elastic foams.
Soft-elastic polyurethane foams are widely used as upholstery materials in the manufacture of seat cushioning and back cushioning. These materials are expected to provide a high degree of comfort comparable with that of natural or latex foams. Physically, the quality of the upholstery may be represented by the so-called "sag" factor (i.e., the quotient of the hardness value at 65% and 25% compression, the overall deformation being kept constant for one minute in either case) and the trend of the force-deformation curves. To obtain good upholstery properties, the sag factor should have a value above 2.5, while the force-deformation curves should not have any plateau. In other words, minimal changes in force should be accompanied by only a minimal change in deformation. Although attempts have been made to improve the sag factor of foams by adding inert fillers, such as barium sulphate or calcium carbonate, the products so produced are attended by serious disadvantages since the fillers are extremely difficult to add and adversely affect the other properties of the foam.
According to German Auslegeschrift Nos. 1,929,034 and 2,002,064, soft-elastic polyurethane foams are obtained by reacting isocyanurate or allophanate polyisocyanates, so-called "modified polyisocyanates", with polyethers containing hydroxyl groups in the presence of amines as catalysts. This necessarily means that the polyisocyanate component has to be modified before the actual foaming process.
According to German Offenlegungsschrift No. 2,003,431, polyurethane foams are obtained by reacting undistilled phosgenation products of primary amines having amine numbers of 87 to 168 with a polyether containing hydroxyl groups in the presence of a non-polymeric polyol as crosslinker, a blowing agent and a catalyst, but in the absence of a silicone stabilizer of the polyoxyalkylene ether-polydimethyl siloxane block copolymer type. This process also has serious disadvantages because the polyisocyanate used as starting material is virtually a crude product which contains different quantities of polyisocyanates differing in their functionality and which does not have any standardized properties. So far as the production of foams is concerned, this means that changes in recipe have to be constantly made, in extreme cases in the amounts of isocyanate used in order to correct for the fluctuations in the activity of the isocyanate.
As already mentioned, "modified isocyanates" are often used for the production of soft-elastic foams. However, the foam manufacturer has to install additional storage tanks for these products and other machinery, inter alia, for their handling and dosage. In the case of a standard commercial-grade "modified isocyanate", the processor cannot himself select or determine the degree of crosslinking of this product. It has now been found that these disadvantages can be obviated by preparing the "modified isocyanate" in a preliminary stage immediately before foaming.